It is well known that cryogenic liquids, or liquids having similar properties, have found great use in industrial refrigeration and freezing applications. For example, liquid carbon dioxide has found use as a commercial refrigerant due to its inert (does not react with plastic) and non-toxic nature and desirable range of refrigeration temperatures. It is typically stored at a pressure of 300 psig and a corresponding equilibrium temperature of approximately 0° F. and then, during dispensing, expanded at atmospheric pressure where it transforms into solid phase CO2 “snow” or dry ice and CO2 vapor. In addition to providing refrigeration, it may also be used in various processes to freeze food items such as hamburger patties or chicken nuggets and the like for shipping and/or storage.
When dispensing the liquid CO2 at pressures around 300 psig, it is known that lowering the temperature below 0° F., in other words, subcooling the liquid, produces a larger percentage of CO2 snow and a smaller percentage of CO2 vapor. As a result, a dispensing system derives higher efficiency by being able to deliver subcooled, high pressure CO2. The corresponding economic advantage increases as the temperature of the liquid CO2 decreases.
In recognition of the above, the system of U.S. Pat. No. 4,888,955 to Tyree, Jr. et al. was developed. The system of the Tyree '955 patent stores liquid CO2 in an insulated tank having a height greater than its internal diameter. A pressure of approximately 300 psig is maintained in the head space of the tank via condensation of vapor therein. Liquid CO2 is withdrawn from the upper portion of the tank and is subcooled outside of the tank by a heat exchanger of an external refrigeration system. The resulting subcooled CO2 liquid is returned to the bottom portion of the tank so that stratification of the CO2 in the tank occurs and a thermocline region is created within the bottom portion of the tank. Subcooled liquid CO2 may then be dispensed from the bottom of the tank due to the approximate 300 psig pressure within the top portion of the tank. The refrigeration system operates during “off hours” to replenish the thermocline region with subcooled CO2.
While the system of the Tyree '955 patent performs well, some food freezing applications do not permit off hours between refills of liquid CO2. It is therefore desirable to provide a system that can operate continuously between refills, and even during refills, of liquid CO2. Furthermore, the ability to reduce the migration of the chilled liquid from the bottom portion of the tank to the warmer liquid in the top portion of the tank, beyond the insulation provided by stratification, would allow the system to operate more efficiently. This would result in less liquid CO2 usage and a smaller compressor in the refrigeration system.